Alkaline dry cells are hitherto used in low-voltage (1.5 V) power sources of strobes, shavers, table clocks, acoustic and electronic equipment and so forth. Positive electrode cans used in such cells are those produced by punching out a coated steel sheet such as a cold-rolled steel sheet or a double-side Ni-coated steel sheet and working the resulting sections by multiple drawing into desired forms (in many instances, cylinders). However, positive electrode cans made of such materials have the problems that their inside surfaces come into contact with high-concentration electrolytes and positive electrode mixtures to form oxide layers on the surfaces of the positive electrode cans, causing a lowering of cell characteristics such as short-circuit current and electromotive force as time lapses.
Now, as a countermeasure for such a problem, a method has been proposed in which the inside surface of a positive electrode is directly coated with a conductive coating composition mixed with a carbon type conductive agent in a large quantity of, e.g., 100 to 300 parts by weight based on 100 parts by weight of resin so that its corrosion resistance can be improved. This is a method in which a carbon type conductive agent as exemplified by graphite powder or acetylene black is mixed into a coating composition comprising a resin with a good alkali resistance such as polyvinyl isobutyl ether or vinyl acetate resin and the resulting coating composition is applied to the inside surface of a positive electrode can (Japanese Patent Applications Laid-open No. 58-48361 and No. 59-160959).
Application of this conductive coating composition, however, must be carried out without pretreatment of positive electrode cans and by a very cumbersome operation of spray-coating or brush-coating them one by one followed by drying, and hence has had a poor productivity. In addition, since the positive electrode cans are small in diameter and nevertheless deep, it has been very difficult from a technical viewpoint to uniformly coat the inside surfaces. Accordingly, it has been sought to provide a material from which positive electrode cans with a good quality can be efficiently produced by only machining.
Inexpensive materials that can meet such demands may include coated steel sheets previously coated with a conductive coating composition. However, the conductive coating composition as mentioned above must be mixed with the conductive agent in a large content and hence may have a poor adhesion of resin to steel sheet to cause separation or break of coating films when worked by drawing.
As other conductive coating compositions, a low electrical resistance coating composition containing graphite powder and carbon black is known in the art (Japanese Patent Publication No. 55-33753). It is disclosed that this coating composition can be made to have a low electrical resistance when graphire powder and carbon black are mixed in an amount of 20 to 1,000 parts by weight in total, based on 100 parts by weight of resin, the proportion of graphire powder to carbon black is controlled in the range of the former/the latter=90/10 to 10/90, and a flaky graphire powder with particle diameters of 0.1 .mu. to scores of microns and carbon black with particle diameters of 10 to 30 m .mu. are used as the graphite powder and the carbon black. It is also disclosed that all sorts of resins can be used for binders of conductive agents so long as they can be usually used as coating materials, which may include thermoplastic resins such as a vinyl chloride-vinyl acetate copolymer and cellulose derivatives and thermosetting resins such as acrylic resins and epoxy resins.
Since, however, this coating composition attaches importance to conductivity, it may cause separation of coating films if the conductive agents are mixed in an amount of 100 to 300 parts by weight based on 100 parts by weight of the resin as in the case of the coating compositions disclosed in Japanese Patent Applications Laid-open No. 58-48361 and No. 59-150959.
A first object of the present invention is to provide a coated metal sheet containing graphite powder or carbon black, or both of them, in a coating film, and from which alkaline dry cell positive electrode cans can be produced by only mschining.
Another object of the present invention is to provide an alkaline dry cell positive electrode can having a superior corrosion resistance.